In the oil and gas industry, downhole tools are used to perform various operations during exploration, production, maintenance or decommissioning. The tools often form part of a tool string that travels downhole, such as a drill string for drilling a bore in an underground formation. Typically the downhole tools perform different functions during different stages of downhole operations. For example, downhole tools are often transported to and from a particular location in a bore and only activated for use at the particular location for a specific interval, such as to perform a local operation such as packing or reaming or perforating, or the like.
It is often unsuitable to transport the downhole tools in an active configuration. For example, there are numerous downhole tools that feature radially extendable members. Blades or cutters such as on an underreamer are radially extendable to allow the underreamer to pass through a restriction or a casing with the blades in a relatively compact radial configuration. When the undereamer passes out of the end of the casing in a bore, the blades are extended to allow the bore to be drilled to a diameter greater than the internal diameter of the casing.
During an underreaming operation the blades can be subjected to high radial forces so, to ensure effective cutting, the blades are radially supported in the extended configuration. Examples of underreamers are described in applicant's International (PCT) Application Publication No.s WO 2004/097163 and WO 2010/116152, the disclosures of which are incorporated herein by reference. Upon completion of an underreaming operation, the blades are retracted to allow the toolstring including the undereamer to be retrieved from the bore. Failure to retract the blades, or to retain the blades in a retracted configuration during retrieval of the underreamer, causes the blades to contact the existing casing. A blade retraction failure of the underreamer makes it difficult, sometimes impossible, to retrieve the underreamer and can also cause damage to the casing or other equipment in the bore.